Delayed coking is a process for the thermal conversion of heavy oils such as petroleum residua (also referred to as “resid”) to produce liquid and vapor hydrocarbon products and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out by converting part of the resids to more valuable hydrocarbon products. The resulting coke has value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
In the delayed coking process, the feed is rapidly heated in a fired heater or tubular furnace. The heated feed is conducted to a coking vessel (also called a “drum”) that is maintained at conditions under which coking occurs, generally at temperatures above about 400° C. and super-atmospheric pressures. The heated feed forms volatile species including hydrocarbons that are removed from the drum overhead and conducted away from the process to, e.g., a fractionator. The process also results in the accumulation of coke in the drum. When the coker drum is full of coke, the heated feed is switched to another drum and hydrocarbon vapors are purged from the coke drum with steam. The drum is then quenched with water to lower the temperature from about 200° F. to about 300° F., after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed after drilling and/or cutting using high velocity water jets. The coke removal step is frequently referred to as “decoking”.
The coke is typically cut from the drum using a high speed, high impact water jet. A hole is typically bored in the coke from water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool cut the coke from the drum. The coke removal step adds considerably to the throughput time of the process. Drilling and removing coke from the drum takes approximately 1 to 6 hours, and the coker drum is not available for feed coking until the coke removal step is completed, which unfavorably impacts the yield of hydrocarbon vapor from the process. Thus, it would be desirable to be able to produce a free-flowing coke, in a coker drum, that would not require the expense and time associated with conventional coke removal.
An additional difficulty that may arise results from the potential for non-uniform coke cooling prior to decoking, a problem sometimes called a “hot drum.” Hot drums occur when, following water quench, regions of the coke volume in the drum remain at a significantly higher temperature than other regions. Hot drum may result during cutting or drilling from the presence of different coke morphologies (e.g., shot and needle, shot and sponge) in different regions of the drum. As a result of the different thermal characteristics among the coke morphologies, some coke regions in the drum may differ in temperature significantly from other regions, which can lead to unpredictable and even hazardous conditions during decoking. Since free-flowing coke morphologies cool faster than agglomerated coke morphologies, it would be desirable to produce predominantly free-lowing coke in a delayed coker, in order to avoid or minimize hot drums.